Bright dip composition for tin/lead

ABSTRACT

A composition for bright dipping tin/lead alloys formed as a coating on substrates of copper so as to avoid tin or lead deposit ion on exposed copper or gold surfaces is provided by an aqueous solution of an acidic compound for mildly etching the tin/lead alloy surface, a thiourea compound and a stabilizer compound which will prevent deposition of tin and lead removed from the coating surface. The acidic compounds are fluoboric acid, tartaric acid, sulfamic acid, phenylsulfonic acid, gluconic acid, or the ammonium or alkali metal salts thereof. The stabilizers are thrichloroacetic acid, and the ammonium and alkali metal salts thereof.

United States Patent [191 Beckwith et al.

[ June 10, 1975 BRIGHT DIP COMPOSITION FOR TIN/LEAD Hsu, Scully Rd, Somers, Conn. 06071 [22] Filed: Mar. 13, 1973 [21] Appl. No.: 340,912

[52] US. Cl. 252/79.3; 156/20; 252/79.4; 252/79.5 [51] Int. Cl C09k 3/00 [58] Field of Search 252/79.1, 79.2, 79.3, 79.4, 252/79.5; 156/18, 20; 134/3, 41

[56] References Cited UNITED STATES PATENTS 3,181,984 5/1965 Jillis 156/20 3,677,949 7/1972 Brindisi et al 252/79.4

Primary Examiner-William A. Powell [57] ABSTRACT A composition for bright dipping tin/lead alloys formed as a coating on substrates of copper so as to avoid tin or lead deposit ion on exposed copper or gold surfaces is provided by an aqueous solution of an acidic compound for mildly etching the tin/lead alloy surface, a thiourea compound and a stabilizer compound which will prevent deposition of tin and lead removed from the coating surface. The acidic compounds are fluoboric acid, tartaric acid, sulfamic acid, phenylsulfonic acid, gluconic acid, or the ammonium or alkali metal salts thereof. The stabilizers are thrichloroacetic acid, and the ammonium and alkali metal salts thereof.

7 Claims, N0 Drawings 1 BRIGHT DIP COMPOSITION FOR TIN/LEAD BACKGROUND OF THE INVENTION Tin/lead alloys are frequently used in the manufacture of printed circuits and other electrical compounds because of their ready solderability and also because of their ability to resist attack by some of the agents employed in the preparation of the circuit boards. Of recent years there has been an increasing desire to develop bright tin/lead electrodeposits so as to afford a desirable cosmetic appearance as well as good electrical properties.

Many chemical compositions are utilized for etching the copper substrate upon which the tin/lead alloys are deposited, and there has been an increasing tendency to utilize alkaline ammoniacal etchants employing chloride ion for this purpose. The use of alkaline ammoniacal etchants frequently develops a dull or tarnished appearance upon the bright tin/lead alloy deposit and there has been an increasing desire for brightening of the deposit following etching of the workpiece.

Various formulations are conventionally employed for the bright dipping of tin/lead alloys and these generally contain an acid for mildly etching the surface of the tin/lead alloy so as to produce a water-soluble salt of the tin and lead, and a thiourea compound Some of the compositions additionally include chloride ion to improve the brightening action and others also include nonionic surface active agents.

However in the course of bright dipping the tin/lead alloy coating, there is a tendency for the tin or lead removed form the coating to chemically deposit upon, and thereby contaminate, the adjacent gold and/or copper surface of the workpiece. Such tin and lead contamination of the surface can materially affect the electrical properties as well as adversely affect the appearance of the workpiece.

Accordingly, it is the object of the present invention to provide a novel bright dip for tin/lead alloys which will effectively eliminate the chemical deposition of tin and lead on adjacent gold and copper surfaces of the workpiece.

It is also an object to provide such a bright dip composition which is relatively economical and which has a relatively long useful life.

SUMMARY OF THE INVENTION It has now been found that the foregoing and related objects can be obtained by use of a composition comprising an aqueous solution, on a per liter basis, of about 50-250 grams of an acidic compound selected from the group consisting of fluoboric acid, tartaric acid, sulfamic acid, phenylsulfonic acid, gluconic acid, the ammonium and alkali metal salts thereof, and mixtures thereof; about -50 grams of a thiourea compound selected from the group consisting of thiourea and monoand di-N-substituted thioureas. Also included are about 0.5- grams of a stabilizer selected from the group consisting of trichloroacetic acid, the ammonium and alkali metal salts thereof, and mixtures thereof. The solution has a pH of about 0.2 to 3.0.

Optional components for the composition include 5-20 grams per liter of a chloride salt selected from the group consisting of alkali metal chlorides, ammonium chlorides, and mixtures thereof, and [-10 grams per liter of a nonionic surface active agent. The preferred compositions utilize sulfamic acid as the acidic compound and thiourea as the thiourea compound;

In the most desirable compositions, the acidic compound is selected from the group consisting of sulfanic acid and fluoboric acid and is present in an amount of -150 grams. The thiourea compound is selected from the group consisting of thiourea and diethylthiourea and is present in an amount of 20-35 grams. The stabilizer is present in an amount of 2-15 grams. Here, the solution desirably has a pH of about 0.5-1.0.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As indicated hereinbefore, the bright dip compositions of the present invention comprise the acidic compound for removing tin/lead alloy and forming a soluble salt thereof, a thiourea compound to promote such removal and a stabilizer to prevent the chemical deposition of tin and lead removed from the coating upon the adjacent exposed surface areas of the gold or copper substrate surface. The compositions may additionally contain chloride ion and a nonionic surface active agent and the pH and temperature may vary.

As indicated above, the acidic compound is one which will provide the desired etching or light removal of the tin/lead surface to effect a brightening action. The acid must be one which will form a relatively soluble salt with the tin and lead ions removed and it must also be one which will not attack the exposed gold or copper surface of the substrate. The acids which have been found suitable are fluoboric acid, tartaric acid, sulfamic acid, phenylsulfonic acid and gluconic acid. These acids may be provided in part as the alkali metal or ammonium salts thereof and generated in solution, particularly upon adjustment of the pH to the desired range. Obviously, it is preferable to add the acidic compounds as the acids themselves to minimize the necessity for pH adjustment. The preferred acidic compounds are sulfamic acid and fluoboric acid which have proven highly effective for commercial applications, and these will normally be present in the range of 50-250 grams per liter and preferably about 80-150 grams per liter.

The thiourea compounds may be provided by thiourea itself or monoand di-N-substituted thiourea such as monoethylthiourea, diethylthiourea, dipropylthiourea, etc. The preferred compounds are thiourea and diethylthiourea. The amount would normally be in the range of 15-50 grams per liter and preferably about 20-35 grams per liter.

The theory of operation of the stabilizers is not completely understood. However, it does appear that they have the effect of reducing the electropotential difference between the tin or lead in solution and the copper or gold surface so as to minimize the tendency of the tin or lead in solution to be reduced and chemically plate out upon the copper or gold surface. The stabilizers include trichloracetic acid, and the alkali metal and ammonium salts thereof, or mixtures thereof. The preferred stabilizers are trichloroacetic acid and its alkali metal or ammonium salts, or mixtures thereof. The amount of stabilizer can vary from a trace to as much as 20 grams per liter. Effectively, 0.5 grams per liter is essential to provide the desired effect for any significant concentration of tin or lead metals in solution. The preferred compositions will contain abour 2-15 grams per liter so as to be effective over substantially the entire useful life of the composition.

The chloride ion has been found useful in improving the quality of bright dipping action and is desirably provided by the addition of alkali metal or ammonium chloride salts. However, hydrochloric acid may be utilized, particularly if the pH of the composition must be adjusted by reason of the addition of the acidic compounds as salts. The chloride salts will normally be present in the range of about 5-20 grams per liter and preferably about 7-14 grams per liter.

In some instances, it is desirable to add a nonionic surface active agent such as the polyoxyalkylene oxide condensates with alkyl phenols. The amount thereof may vary from 1-10 grams per liter depending upon the amount of surface activity that is desired.

The compositions will normally have a pH of 0.2 to 3.0 and perferably about 0.5 to 1.0. The temperature at which they may be employed may vary from as little as to as much as 90 Centigrade. Because of the high contact effect obtained by spray application, temperatures of to Centigrade are preferred for spray use and temperatures of to Centigrade are preferred where the workpieces are dipped in the composition. When spray techniques are employed, the time required for contact between the bright dipping composition and the workpiece will normally range from about 2 to 20 seconds and preferably about 5 to 10 seconds. In immersion or dip applications, the time period will range from 5 to 60 seconds and preferably about 10 to 20 seconds.

Following bright dipping, the workpieces are rinsed in water and spray rinsing is particularly advantageous.

For spray applications, the concentration of the bright-dip formulation is desirably 50 to percent greater than that optimally employed for immersion applications. For example. total concentration of the components will desirably be about 180-300 grams per liter for spray etching applications (preferably about 200-250 grams per liter) whereas for dip applications, it will normally be -220 grams per liter (preferably -175 grams per liter). Since the solutions tend to become depleted over a period of time, it is desirable to make up volume losses by the addition of further solution. If the bright dipping action appears to become less effective, limited additions of the components may be added to maintain the desired brightening action.

In utilizing the compositions of the present invention, the workpieces will normally have undergone prior processing in which a portion of the copper surface is initially provided with an electrodeposit of tin/lead alloy. This may be utilized solely to provide a readily solderable surface portion on the workpiece and/or it may be utilized to function as an etch resist. The workpiece is then subjected to an alkaline etchant to remove copper and it is this etching action which will tend to produce a dull, dark surface upon the tin/lead alloy. This effect is particularly pronounced with ammoniacal etchants containing chloride ion. Following the etching operation, the workpieces should be thoroughly rinsed in water before being exposed to treatment by the bright dip compositions of the present invention. As indicated above, the bright dipping composition may be applied by spray techniques so that the same spray etching equipment employed for etching the workpiece may also function to provide the means of contacting the bright dip compositions with the workpieces.

As will be appreciated, the workpieces being treated must be ones in which bright dipping of the tin/lead alloy is desired and in which there are adjacent exposed surfaces of gold and/or copper. As indicated above, this 5 may occur as a result of plating of the tin/lead upon a copper circuit board and etching of portions of the copper substrate in the ammoniacal etchant. It also may occur when a copper substrate is plated with both gold and tin/lead alloy at different surface portions thereof and the copper is removed at intermediate portions in the alkaline etchant. In the latter instance, the ammoniacal etchant utilized to remove the copper will produce the undesired tarnishing of the tin/lead alloy and the tin and lead metal removed by the bright dip will tend to plate out upon both the copper and the gold if the stabilized composition of the present invention is not employed. This becomes particularly troublesome if the workpiece is one in which the copper surface is not completely etched since the tin or lead chemically deposited on the copper surface will prevent further etching of that portion and requires that the workpiece be scrapped if such further etching is required.

As specific examples of a bright dip formulation in accordance with the present invention, the following are provided indicating further the variation in composition desirable for spray and immersion techniques.

30 Spray Formulation Indicative of the efficacy of the present invention are the following specific examples;

EXAMPLE ONE A stabilized bright dip formulation is prepared in accordance with the immersion formulation recited above. The formulation is maintained at a temperature of 54 Centigrade. A printed circuit board having gold fingers plated upon the copper substrate surface and a tin/lead alloy pattern in other areas is etched with an alkaline ammoniacal etchant to remove exposed copper portions and is thereafter rinsed in water. The etched circuit board exhibits a dark gray tarnish upon the tin/lead deposit.

This board is immersed in the bright dip formulation containing small quantities of tin and lead of the magnitude resulting from treatment of similar workpieces for a period of l0seconds and is then rinsed in water. The tin/lead alloy is found to be bright in appearance and no evidence is seen of any tin or lead deposit upon either the gold or copper surfaces.

A second circuit board is treated in a similar manner except that the bright dip formulation omits the ammonium trichloroacetate. The bright dip formulation is found to produce a bright appearance for the tin/lead alloy coating but there is evidence of tin or lead depositing upon the exposed copper surfaces.

EXAMPLE TWO A stabilized bright dip formulation is prepared in accordance with the following formulation:

Fluohoric acid 1 grams Thiourea 40 grams Ammonium trichloroacctate 13 grams Water to one liter Circuit boards are treated as in Example One and those immersed in the unstabilized composition are found to evidence a tin or lead deposit on the exposed copper surfaces whereas those treated in the stabilized formulation do not evidence such a tin or lead deposit.

Thus it can be seen from the foregoing detailed description and examples that the present invention provides a novel bright dip for tin/lead alloys which effectively eliminates the chemical deposition of tin or lead on adjacent gold and copper surfaces of the workpiece. The bright dip composition is relatively economical and has a relatively long useful life.

Having thus described the invention. we claim:

1. A composition for bright dipping tin/lead alloys on a workpiece having exposed surfaces of copper or gold without contamination thereof comprising an aqueous solution, on a per liter basis. of:

A. about 50-250 grams of an acidic compound selected from the group consisting of fluoboric acid, tartaric acid, sulfamic acid, phenylsulfonic acid,

gluconic acid, the ammonium and alkali metal salts thereof, and mixtures thereof;

B. about 15-50 grams of a thiourea compound selected from the group consisting of thiourea and monoand di-N-substituted thioureas and mixtures thereof; and;

C. about 0.5-20 grams of a stabilizer selected from the group consisting of trichloroacetic acid, the ammonium and alkali metal salts thereof, and mixtures thereof;

said solution having a pH of about 0.2 to 3.0.

2. The composition of claim 1 further including 5-20 grams of a chloride salt selected from the group consisting of alkali metal chlorides, ammonium chlorides, and mixtures thereof.

3. The composition of claim 1 wherein said acidic compound is sulfamic acid.

4. The composition of claim 1 wherein there is included 1-10 grams of a nonionic surface active agent.

5. The composition of claim 1 wherein said thiourea compound is thiourea.

6. The composition of claim 1 wherein said acidic compound is selected from the group consisting of sulfamic acid and fluoboric acid and is present in an amount of -150 grams, wherein said thiourea compound is selected from the group consisting of thiourea and diethylthiourea and is present in an amount of 20-35 grams, and wherein said stabilizer is present in an amount of 2-1 5 grams and wherein said solution has a pH of about 0.5-1.0.

7. The composition of claim 6 wherein there is included 5-20 grams of a chloride salt selected from the group consisting of alkali metal chlorides. ammonium chlorides, and mixtures thereof. 

1. A COMPOSITION FOR BRIGHT DIPPING TIN/LEAD ALLOYS ON A WORKPIECE HAVING EXPOSED SURFACES OF COPPER OR GOLD WITHOUT CONTAMINATION THEREOF COMPRISISNG AN AQUEOUS SOLUTION, ON A PER LITERS BASIS OF: A. ABOUT 50-250 GRAMS OF AN ACIDIC COMPOUND SELECTED FROM THE GROUP CONSISITNG OF FLUOBORIC ACID, TARTARIC ACID, SULFAMIC ACID, PHENYLSULFONIC ACID, GLUCONIC ACID, THE AMMONIUM AND ALKALI METAL SALTS THEREOF, AND MIXTURES THEREOF; B. ABOUT 15-50 GRAMS OF A THIOUREA COMPOUND SELECTED FROM THE GROUP CONSISTING OF THIOUREA AND MONO- AND DI-N-SUBSTITUTED THIOUREAS AND MIXTURES THEREOF; AND C. ABOUT 0.5-20 GRAMS OF A STABLIZER SELECTED FROM THE GROUP CONSISTING OF TRICHLOROACWTIC ACID, THE AMMONIUM AND ALKALI METAL SALTS THEREOF AND MIXTURES THEREOF; SAID SOLUTION HAVING A PH OF ABOUT 0.2 TO 3.0.
 2. The composition of claim 1 further including 5-20 grams of a chloride salt selected from the group consisting of alkali metal chlorides, ammonium chlorides, and mixtures thereof.
 3. The composition of claim 1 wherein said acidic compound is sulfamic acid.
 4. The composition of claim 1 wherein there is included 1-10 grams of a nonionic surface active agent.
 5. The composition of claim 1 wherein said thiourea compound is thiourea.
 6. The composition of claim 1 wherein said acidic compound is selected from the group consisting of sulfamic acid and fluoboric acid and is present in an amount of 80-150 grams, wherein said thiourea compound is selected from the group consisting of thiourea and diethylthiourea and is present in an amount of 20-35 grams, and wherein said stabilizer is present in an amount of 2-15 grams and wherein said solution has a pH of about 0.5-1.0.
 7. The composition of claim 6 wherein there is included 5-20 grams of a chloride salt selected from the group consisting of alkali metal chlorides, ammonium chlorides, and mixtures thereof. 